LIPO Batteries – Charging – Discharging – Storing –

 

LiPo Battery (Lithium Polymer)

A type of battery used nowadays in thousands of devices, tools, models, transmitters, and too numerous to list, in countless places. These batteries have played a significant role in the RC industry in the past few years and remain an excellent choice for those seeking high-performance and long-term usable energy sources. There is a wide selection of LiPo batteries, where everyone can find the right power source for themselves.

The first question: which battery type to choose? LiPo vs NiMH

LiPo battery:

Advantages:

• Much lighter, can be made in virtually any size or shape
• Much greater capacity, can store much more energy
• Better discharge rate, can release energy much faster

Disadvantages:

• Short lifespan, maximum 150-250 charge cycles
• Sensitive, can cause fires
• Charging, discharging, storage only possible with proper tools and care

NiMH Batteries:

Advantages:

• Longer lifespan, usually withstands 1000 charge cycles
• Not sensitive, usually not fire hazardous
• Easy to use, simple chargers suffice, and no need for special care

Disadvantages:

• Heavier, and their size is limited
• Low discharge rate, cannot deliver large amounts of current at once
• Lower average capacity, less efficient

 

 

What do the numbers and markings on a LiPo battery mean?

Discharge (C) rating (discharge speed):

The C-rate is a measure of the safe discharge speed according to the battery’s maximum capacity. This is the maximum rate of discharge that the battery can handle without damage. A 1C rate means that the discharge current can deplete the entire battery within one hour. For a battery with a 100 amp capacity, this is equivalent to a 100 amp discharge current.

50C = 50 x capacity (in amperes) e.g., for a 5000 mAh battery, this is 50 x 5A = 250A, the maximum current (load) it can deliver without damage. If the current intake is higher (e.g., a poorly chosen motor, propeller, battery), it can lead to battery damage and even fire.

Capacity:

The battery’s capacity indicates how much energy the battery can essentially hold. It’s somewhat like the size of a fuel tank. The unit here is milliamp hours (mAh). This means how much energy it can deliver in one hour.

As mentioned above, the battery’s capacity is like a car’s fuel tank, thus the capacity determines how long it can run before needing to be recharged. The larger this number, the longer it can run. Capacities cover a wide range from 100mAh to 12000mAh. There is also a drawback to capacity. The larger the capacity, the larger the battery’s weight and physical size.

Voltage, cell count (Voltage / Cell count):

The nominal voltage of a LiPo cell is 3.7V. For the 7.4V battery shown in the picture, this means that two cells are connected in series (i.e., the voltage adds up). Thus, the 2S marking on the battery indicates just what has been described above. So, 2 cells (2S), 3 cells (3S), 4 cells (4s), and so on.

Nominal voltage:

The nominal voltage is the battery’s default rest voltage (average value set by manufacturers). However, this is not the charging voltage. The maximum charging voltage for LiPo batteries is 4.2V, and the safe minimum voltage is 3V (3.2V), with 3.7V being about the midpoint.
Previously, you might have encountered the 2S2P marking. This is actually a 4-cell battery, where 2 cells are connected in series (voltage adds up) and two other cells are connected in parallel to the previous cells (their capacities add up).

The following article on modifying and connecting LiPo batteries details how a multi-cell LiPo battery is constructed, how we can measure it, and even modify it.

Internal resistance of LiPo batteries:

There’s a very important classification we haven’t talked about yet. This is the Internal Resistance (IR). Unfortunately, we will not find this data on the battery. The reason for this is that this value changes over time, and can also change due to external conditions, e.g., temperature. In a sense, the internal resistance is one of the most important parameters of batteries. To understand why this is important we need to understand what it is. Simply put, internal resistance means, for example, how difficult it is to get the energy to the consumer, e.g., to the motor, or anything else that is connected to the battery.

The larger this value, the more difficult it is for the energy to reach its destination, and the energy needed to overcome this resistance is lost as heat energy, also known as heat loss. The internal resistance, therefore, is a measure of the battery’s efficiency. This value is measured in milliohms (mΩ). (1000 mΩ = 1 Ω)

Care and handling of LiPo batteries:

Charging LiPo batteries:

The first and extremely important thing is that a charger suitable for charging LiPo batteries must be used. As mentioned above, these batteries require special care, and the so-called CC/CV (Constant Current / Constant Voltage) method. Basically, the charger maintains the current or charging speed until the battery reaches the peak voltage (4.2V per cell), then maintains this voltage while reducing the current strength.

However, NiMH and NiCd batteries are typically charged with pulse charging.

The second reason why it matters what kind of charger we use is Balancing – leveling (Balance). Balancing is the equalization of the individual cell voltages of the battery, trying to bring each cell to the same voltage. This helps to maintain battery performance and is also important from a safety standpoint. Multi-cell LiPo batteries are equipped with balance connectors which are JST-XH connectors. Through these, the charger can control, and balance the individual cell voltages.

LiPo batteries should be charged slower than NiMH, NiCd batteries. The general rule is that batteries should be charged at a 1C charging rate. So, for example, a 3000mAh battery should be charged at 3A, a 500mAh battery at 0.5A, a 4500mAh => 4.5A. However, nowadays more and more LiPo batteries are appearing that can handle charging rates higher than 1C, e.g., “3C Charge rate” which means for a 5000mAh battery the maximum charging current is 3 x 5A = 15A.

Due to the fire hazard associated with the use of LiPo batteries, regardless of the likelihood, appropriate precautions must always be taken.

• Never leave the charger unattended during charging.
• Always have a fire extinguisher nearby. (A CO2 carbon dioxide fire extinguisher is ideal in this case)
• Charge and store LiPo batteries in a fireproof place, in a bag, in a box.

Parallel vs series charging: charging multiple batteries in parallel or in series.

Parallel charging: Not allowed!

Manufacturers do not recommend it and consider it dangerous. The problem with this method is that if the batteries are connected to each other in parallel, the charger cannot see the parallel-connected cells separately, but can only measure one voltage. This automatically results in overcharging.

The other problem is the uniqueness of the batteries. If two batteries (and the cells within them) come from exactly the same production batch, have exactly the same chemical composition, age, state of charge, and everything else, in other words, if they are completely identical then this method might be conceivable, but this does not occur in reality.

Series charging:

If the batteries are single-cell and have the same capacity then they can be connected in series, and the charger can be set as if it were a 6-cell LiPo and with balance charging, the charger can be induced to charge each cell separately, ensuring that each is kept at a safe level. If you only have a single port charger then series charging is the only viable option, but the proper wiring is quite complicated and not something a beginner should attempt.

The absolute best way is to purchase a multi-port charger and use it.

Discharge:

LiPo batteries are extremely sensitive, unfortunately, this is caused by the chemistry of the battery itself. The batteries contain lithium, alkali metals, which react with water and burn at high temperatures. During the use of the batteries, Li2O lithium oxide precipitates at the cathode and anode, which is essentially lithium corrosion – rust. This rust increases the internal resistance, resulting in the battery heating up more and more. Due to the heat, an excess of oxygen accumulates inside the battery, leading to the well-known swelling. Unfortunately, this is the moment when we have to say that the particular battery can no longer be used. Further use generates more heat, and thus the thermal runaway may occur, a self-sustaining process that accelerates the heating process, releasing more energy, which further increases the temperature. Basically, when this reaction starts, it generates heat, which increases the resistance (more Li2O) which increases the resistance, which generates more heat, oxygen is released, which increases the swelling. This continues until the casing ruptures, at which point the lithium, meeting the moisture in the air, ignites and causes a fire.

The charging/discharging cycle of LiPo batteries in an ideal world is 300-400 charges/discharge, in reality, it’s about 150-250 cycles. In every case, during use, or when the voltage drops below 3.0v, or if damaged, in any way water, water vapor gets into the battery, it reduces the lifespan of the LiPo and accelerates the growth of Li2O. Therefore, LiPo batteries should not be discharged below 3.0v.

 

Storing LiPo batteries

Safe ways to store LiPo (lithium-polymer) batteries are very important to avoid accidents and ensure a long lifespan for these energy storage devices. Here are some basic information about storing LiPo batteries in an understandable way:

1. In a cool and dry place:
   • Store the LiPo batteries in a cool, dry place where there are no temperature fluctuations and minimal humidity.
2. Away from heat sources:
   • Avoid storing LiPo batteries in direct sunlight or near heat sources. High temperatures can damage the batteries.
3. Safety bag or box:
   • Store the LiPo batteries in a safety bag or box to minimize damage in the event of an accident.
4. Charge level:
   • Do not store the LiPo batteries fully charged or completely discharged. Ideally, they should be stored at a 40-60% charge level for long-term inactivity. This is usually 3.8V per cell.
5. Proper packaging:
   • If you need to transport or store LiPo batteries, ensure proper packaging to minimize damage and risk of accidents.
6. Regular inspection:
   • Periodically check the LiPo batteries for any damage or abnormalities. If you notice any problems, do not continue to use them and consult an expert.
7. Keep away from children:
   • LiPo batteries consist of small parts that can be dangerous if handled improperly. Store them in a place where young children cannot access them.

These basic steps help ensure the safe storage of LiPo batteries and contribute to their longer lifespan. It is important to always follow the manufacturer’s recommended storage and handling instructions.

 

Regenerating Lipo batteries:

Unfortunately, the regeneration of lithium-polymer (LiPo) batteries is not widely accepted or recommended practice. LiPo batteries have special handling, and there is no reliable, widely accepted method for their regeneration.

LiPo batteries are generally designed to operate safely and efficiently throughout their lifespan. However, over time and through charge cycles, they tend to wear out. If problems arise, such as a decrease in their capacity or unstable performance, the recommended step is usually to procure a new battery.

Some general tips, however, for optimizing the lifespan of LiPo batteries:

1. Proper charging and discharging cycles: Avoid overcharging and complete discharge. Keep the battery within the range recommended by the manufacturer.
2. Appropriate temperature: Do not use the battery under extreme temperature conditions as it can damage it.
3. Storage: If you do not use it for an extended period, store the battery at the recommended charge level.
4. Safe use: Avoid mechanical damage and fire hazard, do not leave unattended while charging.

If the battery’s performance significantly decreases or problems arise, the best solution may be to procure a new LiPo battery. Regeneration attempts can not only be dangerous but also do not guarantee satisfactory results. Always follow the manufacturer’s instructions and warnings for safe use.